The Silver Sea by Julia Golding

Freydis has been left for dead following a raid on her father’s stronghold Her brother has been taken prisoner. Her father, a Viking leader, is bent on revenge and takes her on his quest, giving her Enno—a most unusual slave—to protect her. As the hunt for her brother becomes more desperate, Freydis and Enno draw closer together. But when battle looms, the bonds of friendship, family, and loyalty are tested to their limits.